The present invention relates the field of terrestrial telescopes, and more particularly, to a compact collapsible terrestrial telescope employing an objective lens and a terrestrial eyepiece, wherein both the objective lens and the terrestrial eyepiece include an aspheric surface.
Terrestrial telescopes are distinguished from astronomical telescopes by the production of an erect image. That is, terrestrial telescopes provide an erect image for observation in combination with angular magnification of the object field.
Terrestrial telescopes may be generally divided into five classes. The first class is a Galilean telescope in which an objective lens group is separated from an eyepiece lens group by a relatively large intervening air space. No intermediate image exists in the Galilean telescope and magnification is limited to less than approximately 3.times. for a useful (approximately 4 degrees) full object field diameter. Galilean telescopes are often referred to as "opera glasses." However, image quality is limited and the devices have a small field of view and minimal magnifying power.
The second class of terrestrial telescopes includes the Keplerian telescope with erecting prisms. Many binoculars and spotting telescopes employ this design. This Keplerian telescope includes an objective group and an eyepiece group, wherein the focal point of the objective group coincides with the focal point of the eyepiece group to produce an intermediate image. A set of erecting prisms (e.g. porro or roof prisms) is placed near the intermediate image to erect the image. The Keplerian telescope provides substantial angular magnification, up to 20X and the field of view can be as large as 7 degrees for a 7X instrument. However, the prisms add undesired weight and expense to the telescope.
The third group of terrestrial telescopes includes the Keplerian telescope with a symmetric erecting relay. The erect image in this Keplerian telescope is formed by an image forming relay between the objective lens group and the eyepiece lens group. This Keplerian telescope forms two intermediate images. The image forming relay includes two lens groups which are identical and separated by the sum of their focal points with an image of the aperture stop midway between the lens groups. That is, the image forming relay is symmetrical about the aperture stop. U.S. Pat. No. 4,367,010 is exemplary of the symmetric Keplerian telescope.
A fourth class of terrestrial telescopes includes a Keplerian telescope with extended eye relief. In the extended Keplerian telescope, two erecting relay elements are located adjacent to each other so that the aperture stop image lies outside the relay group. While the extended Keplerian telescope can be relatively short, thereby maintaining relatively long eye relief, the diameter and weight of the relay elements increases both cost and weight of the device.
The final class of terrestrial telescopes is the Keplerian telescope with a terrestrial eyepiece. The terrestrial eyepiece employs four element groups to erect the image and present the image for viewing. The first three elements of the eyepiece are typically plano convex elements and form an erector. The remaining element presents the image for viewing. As optical glass elements with spherical surfaces operate over a very limited (typically 2 degrees) full object field and have large optical aberrations that limit image resolution, the terrestrial eyepiece has long been disfavored.
Therefore, a need exists for a cost efficient collapsible terrestrial telescope capable of producing a high quality image. In addition, the need exists for a collapsible terrestrial telescope which provides an adequate field of view and magnification without requiring excessive weight.